A class of catalysts in present use for a variety of reactions is the metal and metal oxide class usually comprising a metal or metal oxide deposited on a support and characterized by having a high surface area. Generally, such catalysts are prepared by mixing the appropriate metal or metal oxide with an inert filler and compacting the resulting mixture to the desired size or shape. Alternatively, they may be prepared by mixing a solution of the appropriate metal salt with a suitable support wherein the salt is absorbed on the surface of the support. After solvent removal, the support is heated in an oxidizing atmosphere to convert the metal salt to the oxide form or alternatively in a reducing atmosphere to convert the metal salt to the free metal. Normally, the solution process gives a catalyst of high surface area, provided the support has a high surface area.
This invention relates to a novel process for the preparation of iron oxide catalysts by adsorbing iron pentacarbonyl upon a suitable support, and then oxidizing the iron pentacarbonyl to an amorphous iron oxide. The many uses for such iron oxide catalysts include use in the dehydrogenation of butylene to butadiene, the conversion of ethylbenzene to styrene, the synthesis of ammonia and the removal of nitrogen oxides from exhaust gases.
The preparation of metallic iron catalysts by decomposing iron pentacarbonyl is not new. Vesterdal, U.S. Pat. No. 2,533,071, discloses the preparation of metallic iron catalysts by heating iron pentacarbonyl on a support to a temperature of about 400 - 1000.degree.F at 1 to 30 atmospheres to degrade the iron pentacarbonyl to metallic iron. Garwood et al, U.S. Pat. No. 3,297,565, in Example 5, treat, at 395.degree.-480.degree.F, a support with iron pentacarbonyl in light distillate fuel and then decompose the pentacarbonyl to metallic iron by heating to 480.degree.-645.degree.F in a helium atmosphere. The decomposition of iron pentacarbonyl to iron oxide is also old. Chem. Abs. 46 (1952) 10994.sup. h discloses the preparation of crystalline iron oxides from iron pentacarbonyl and carbon monoxide. Chem. Abs. 47 (1953) 8484.sup.e discloses the decomposition of the pentacarbonyl by activations caused by adsorption on the walls of the reaction vessel or by illumination. Chem. Abs. 67 (1967) 45635.sup.k discloses the vapor phase preparation of highly dispersed iron oxide by contacting a mixture of iron pentacarbonyl in a carrier gas with a tangentially blown stream of an oxygen containing gas.
The prior art has not however recognized the benefits to be obtained by using iron pentacarbonyl as the starting material for the preparation of a highly active supported amorphous iron oxide catalyst. It is well known that chemical composition is often a poor guide in predicting the activity of a catalyst. More often than not the physical characteristics of a porous solid catalyst such as particle size, pore size, surface area and crystal structure determine efficiency as well as selectivity. In general, both chemical and physical properties together govern the activity of a catalyst, and this is determined by the method of preparation and pretreatment.
It is thus an object of this invention to prepare a highly active, amorphous, efficient and selective iron oxide catalyst, having uniformly small particle size and high surface area. Other objects will become apparent on reading further.
Our novel highly active iron oxide catalysts are prepared by vaporizing an iron carbonyl onto a suitable support using a gas as an inert carrier and oxidizing the adsorbed iron pentacarbonyl to produce an amorphous iron oxide catalyst. Our catalyst is highly active and gives improved results when used in the dehydrogenation of butylene to butadiene, the conversion of ethylbenzene to styrene and the synthesis of ammonia. It gives especially good results when used to remove nitrogen oxides from exhaust gases.